The present invention relates to sheet-fed printing machines and, more particularly to an improved stack changing device for sheet feeders of sheet-fed printing machines.
It is a known practice, in sheet feeders for sheet-fed printing presses or other sheet-processing machines, to provide arrangements for automated stack change. These may consist of rack-type structures, so-called remaining-stack carrying devices, which are provided with thrusting and lifting drives for the horizontal and vertical movement. Such so-called non-stop stack changers are suited, for example, during the printing of paper sheets, i.e. in machine running, to take over remainders of finished sheet stacks from, for example, a pallet with grooves, and to deposit them again on a new sheet stack subsequently installed in the sheet feeder. Known devices are distinguished by high constructive and assembly expenditure and require special constructions of the sheet feeders. Further, devices are used here, the remaining-stack carrying device of which have a rack engaging into the grooves of the pallet. This rack is to be removed in the joining of the remaining stack with the newly installed sheet stack as a whole between the two stack parts. This involves high drive forces and places very strong stresses on the sheets lying next to the section point. Furthermore, restraining means are to be provided which prevent a shifting of the stack parts, and, in the process, severely strain the stack edges. Furthermore, the operation of the sheet feeder itself is severely hindered or even rendered impossible. The sheet flow is difficult to control in the changing operation, so that waste sheets result again and again.
Devices have already been developed that partly avoid some of the disadvantages described.
Thus, from DE 393 1710 C2 there is known a non-stop sheet feeder for sheet-fed rotary presses. It has a remaining-stack carrying device which is arranged underneath a conveyor table leading from the sheet feeder to the sheet-fed rotary press. The remaining-stack carrying device has a closed frame on which non-stop bars are arranged, which can be driven as piston rods of individual cylinders by means of a pressure medium, and which are drivable into grooves of a pallet carrying a sheet stack. The non-stop rods lie, in the driven-in state, on both sides of the frame and are to be removed successively from the zone of the sheet feeder. While the rods have individual drive cylinders, the patent discloses nothing about the sequence of operation. The bridging of the gap conditioned by the non-stop bars between main stack and remaining stack is an obstacle to a faultless continuous processing in the unification of the stacks.
From DE 4 203 500 A1 a sheet feeder is known. It has, parallel to the sheet feeder and allocated to this on the face side, an auxiliary stack-carrying device as an independent component. Then, over a common drive, individually drivable pointed bars are provided which can be introduced into grooves of a pallet carrying a sheet stack. The drive has individual chain gears which are couplable onto the respective pointed bars. For the guidance and accessibility of the chain drives, special constructive measures are required. The chain drives completely block the space in front of the sheet feeder, so that the latter is not accessible. In the stack changing, it is provided to remove the pointed bars in the joining of main stack and remaining stack, first on the outside, then in the middle and lastly in the zone between the already pulled pointed bars out of the stack and remaining stack, so that a gentle depositing of the remaining stack of the sheet stack results. This, however, is possible with the requisite precision only in the case of heavy materials such as sheets of metal, since the sheets bulge in different direction and must sink over a large gap that is formed by the pointed bars.
Finally, from DE 19520772 C1 there is known a non-stop sheet feeder for printing presses with fork bars movable in and out. In this feeder on both sides of the stack zone there are provided units of fork bars movable transversely to the sheet transport direction. The fork bars are joined with one another and are drivable in common into grooves of a pallet carrying a sheet stack. From there they can take over an auxiliary stack in order to bridge the time period up to the supplying of a new sheet stack. For the preliminary approach of the main stack and of the remaining stack before the final joining, the fork bars are rectangular in cross section and turnable about a longitudinal axis. There the fork bars thrust in first in the upright orientation, in order to be able to carry as great a load as possible. For the approach they are then rotated through 90 degrees, so that the main stack and the remaining stack approach the value of the thickness of the narrower side to each other. In the turning of the fork bars measures have to be taken in order to avoid a shifting of the sheets lying next to the fork bars. This has proved in practice to be virtually unfeasible.
Further, from DAS 105 297 there is known a sheet feeder with several stack-lifting mechanisms. It has a fork-shaped remaining-stack carrying device, which is provided with remaining-stack bars slidable into grooves of a pallet. The device makes possible the take-over of a remainder of a sheet stack from the pallet for the continuous feeding of the sheets while a new sheet stack is installed into the sheet feeder. The remaining-stack device is connected with a separate lifting mechanism parallel to the main-stack lifting mechanism inside the sheet feeder, so that the remaining stack is continuously liftable. The operating range of the remaining-stack carrying device is restricted. The remaining-stack carrying device hampers access to the sheet feeder.
In view of the foregoing, an object of the present invention is to provide an improved stack changing device which overcomes the problems associated with prior art designs.
A more specific object of the present invention is to provide a stack changing device which allows for simple and continuous stack changing with undisturbed sheet transport and no wasted paper.
The present invention provides these and other advantages and overcomes the drawbacks of the prior art by providing a stack changing device which utilizes an improved method and device for receiving a remaining-stack and transferring the remaining stack to a newly fed-in sheet stack.
It is advantageous that carrying and spacing bars movable independently from one another are provided in the device, which are pulled out of the stack zone not simultaneously but intermittently for the unburdening of the sheet material. There can be provided preferably a different height of the two types of bars, which generates a smooth depositing movement of the remaining stack on the sheet stack. The pulling movement of the bars has no influence on the remaining stack resting on it, so that the directly affected paper sheets are spared, since only slight or no retaining forces are required. In particular the continuous removal of the remaining-stack bars from inward, outward and in two states makes possible a gentle depositing of the remaining stack on the sheet stack. Moreover, it is advantageous to make the remaining-stack bars, according to type, of different length, so that an improved working run-off results.
These and other features and advantages of the invention will be more readily apparent upon reading the following description of preferred exemplary embodiments of the invention and upon reference to the drawings wherein: